1. Field of the Invention
The present invention generally relates to an image forming device. More specifically, the present invention relates to an image forming device such as a photocopier, printer, or the like.
2. Background Information
Normally in an image forming device, an electrostatic latent image is formed on a photosensitive drum, which is the image carrier. This electrostatic latent image is developed in a developing device to become a toner image. The toner image on the photosensitive drum is transferred onto a recording sheet. Then the toner image that is transferred onto the recording sheet is fixed onto the recording sheet by a fixing device.
Some developing devices use a two component developer that includes carrier and toner. In this type of developing device, when an image is being formed only toner is consumed, so the mixing ratio of toner and carrier varies. In order to obtain stable images, it is necessary to replenish the toner to maintain the mixing ratio of toner and carrier within a fixed range. Therefore in this type of developing device, the concentration of toner in the developer is measured with a magnetic sensor, and the toner is replenished based on the measured toner concentration.
Conventionally, the toner concentration is measured using the average value of sensor output voltage of a magnetic sensor as a characteristic value of sensor output as shown in Japanese Patent Application Laid-open No. H10-186833. Also, there are magnetic sensors that measure the minimum value in one period of the sensor output wave form as shown in Japanese Patent Application Laid-open No. 2001-354864. However, the method of measuring the toner concentration disclosed in Japanese Patent Application Laid-open No. 2001-354864 is valid only for developing devices having a special agitation member. The method is not valid for developing devices with the commonly-used screw-shaped agitation member.
In developing devices with the commonly used screw-shaped agitation member, the bulk density and consistency characteristics of the developer that is transported varies due to degradation with time and humidity and other environmental conditions. This also results in a change in the sensor output of the magnetic sensors. Also, when it is necessary to change the concentration of toner, the sensor output of the magnetic sensor also changes.
In these cases the shape of the sensor output wave form has the period of the agitation transport roller. However, when the sensor output is changed as referred to above, the sensor output wave form is not simply changed by shifting the output wave form, the minimum value of the sensor output wave form does not change, but the sensor output wave form is changed up to the maximum value. Therefore, using the average value of sensor output as the sensor characteristic output value to control the replenishment of toner does not achieve a stable toner concentration.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved image forming device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.